For portable devices such as cellular phone cameras and laptop computer cameras, it is desirable to reduce costs as much as possible and to keep the thickness of the portable device as thin as possible. Folding the optical path of a lens is an excellent way to change the form factor of the optical assembly. Consequently, a folded optical assembly is an attractive alternative for the image capture device in a portable apparatus. It is the goal of the present invention to provide a low cost folded optical assembly for an image capture device wherein the thickness of the optical assembly is less than 7 mm.
Examples of folded optical assemblies can be found in U.S. Pat. Nos. 6,898,023 (Takeuchi), 6,900,950 (Nagata), and U.S. Patent Application Publication No. 2006/0092524 (Konno). Takeuchi discloses a zoom lens assembly with a series of refractive elements and a prism to fold the optical path. Takeuchi's folded zoom lens assembly is over 7 mm in thickness and it would be relatively expensive since it is composed of five lenses, one prism, and one filter. In addition, the third lens group moves as the lens zooms so guidance is required on this lens group. As a result, the zoom lens assembly described by Takeuchi is relatively thick and relatively expensive.
Nagata discloses a folded lens assembly, shown in FIG. 1, that is composed of a series of two refractive surfaces (100 and 160) and three reflective surfaces (110, 140, and 150) to create an optical path with three folds. Refractive surfaces 120 and 130 are typically parallel surfaces and as such do not have optical power but they could have refractive functionality. Element 170 is a cover glass and infrared filter which does not have optical power. A problem with the design described by Nagata is that multiple free form surfaces are used. The free form surfaces are required to generate optical power from the three curved reflective surfaces (110, 140, and 150) used in the Nagata lens design to focus the image on the sensor 180. Free form surfaces are not rotationally symmetric and as such the mold components for molding the lenses are not manufacturable with rotationally based tooling processes such as diamond turning or traditional grinding and polishing so that manufacturing costs are substantially increased.
Konno discloses a folded zoom lens assembly, shown in FIG. 2, that includes a series of five refractive lenses (230, 240, 250, 255 and 260) and two prism elements (210 and 270) to fold the optical path as and focus an image on the sensor 285. Element 275 is a cover glass and infrared filter which does not have optical power. This folded zoom lens assembly would also be relatively expensive since the embodiments described include glass lens elements and prisms. In addition, the lens elements between the two prisms are movable to provide the zoom function which requires complicated guidance mechanisms.
Therefore a need persists for a low cost folded optical assembly for an image capture device wherein the thickness dimension of the optical assembly is thin.